Investigation of the unusual magnetic properties of Fe and Co on MgO with high spatial, energy and temporal resolution

Nanometer-sized magnets are used as magnetic bits in data storage devices to hold information. As such devices are further miniaturized, the control of magnetism at the atomic scale becomes essential. One of the critical parameters to realize nanoscopic magnets is a large magnetic anisotropy. Magnetic anisotropy gives its magnetization a preferred axis and thus its directionality. At the atomic scale, magnetic anisotropy originates from anisotropy in the orbital angular momentum and the spin-orbit coupling that connects the spin moment of a magnetic atom to the spatial symmetry of its ligand field environment. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. Here we investigate the magnetic properties of individual magnetic atoms on thin magnesium oxide (MgO) films. We find highly unusual magnetic behavior for Fe and Co on the oxygen binding site of MgO. By coordinating a single Co atom to this binding site we can even realized the maximum magnetic anisotropy for a 3d transition metal atom.
\\
At the heart of this work we combine scanning tunneling microscopy and X-ray absorption spectroscopy experiments and we find striking agreement between these experimental techniques. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts for Co as well as 14 millielectron volts for Fe on the oxygen binding site. This behavior originates from the dominating axial ligand field of this adsorption site, which leads to out-of-plane uniaxial anisotropy combined with large orbital moment, as observed by X-ray magnetic circular dichroism. The bonding geometry and electronic configuration are calculated by density functional theory, a multiplet analysis and a model developed here, that uses a point-charge calculation combined with Stevens operator equivalents. Scanning tunneling microscopy also allows the tuning of the magnetic anisotropy and spin-polarized measurements that confirm the applied model by revealing further transitions and by allowing the measurement of magnetic moments on single atoms.
\\
A further critical parameter for obtaining miniaturized magnets, for applications in data storage devices, is the magnetic stability, the ability of magnets to retain their magnetic orientation despite external influences. The magnetic stability of larger magnets is determined by a thermal barrier, which scales with the magnetic anisotropy. At the atomic scale, magnetization reversal is often dominated by quantum tunneling of the magnetization. Quantum tunneling allows transitions between states without having to overcome the anisotropy barrier. This is for example caused by mixing between different states, induced by the ligand symmetry. Here we use an all-electrical pump-probe scheme to determine the lifetimes of Co and Fe on MgO and we show how such tunneling can be sufficiently suppressed by careful design of the bonding geometry and by reducing the atom’s interaction with the environment. With this approach, we show the longest lifetimes seen so far for 3d transition metal atoms: a lifetime of 200 microseconds for Co and of 10 milliseconds for Fe.
\\
The research described in this thesis demonstrates how the complementary use of several experimental and theoretical techniques allows a detailed determination of the character and properties of the magnetic states at the atomic level. These results offer a strategy, based on symmetry arguments and careful tailoring of the interaction with the environment, for the rational design of nanoscopic permanent magnets and single atom magnets.